1. Field of the Invention
This invention relates to calcium tungstate phosphors suitable for X-ray usage and to processes for preparing same.
2. Prior Art
Calcium tungstate CaWO.sub.4, is a phosphor material that luminesces efficiently under X-ray excitation. Such phosphor materials are embedded in a binder and manufactured into a screen. The screen is used to intensify the image produced on a photographic film in X-ray machines. Brightness is an important factor in minimizing the patient's exposure to X-rays.
Several methods for preparing CaWO.sub.4 are known. The fusion or solid state methods involve heating together at elevated temperature a calcium source and a tungstate source. Typical methods include heating a calcium compound such as calcium carbonate and tungsten trioxide. In such reactions, carbon dioxide is evolved. Another solid state process is to heat a calcium halide such as calcium chloride with a tungstate salt such as sodium tungstate.
In U.S. Pat. No. 1,602,593, issued Oct. 12, 1926, lag is stated to be reduced by adding vanadium salts at the 0.2 to 0.5 percent vanadate by weight of the calcium tungstate produced by solid state techniques. Complete elimination of lag is stated to be not achieved. In more recent years, purity of phosphor raw materials and of the phosphors have been improved and generally addition of other materials above about 500 ppm are avoided.
Chemical precipitation method is a known method for preparing calcium tungstate. In this method, an aqueous solution of a water-soluble calcium compound such as calcium chloride and a water-soluble tungstate salt such as sodium tungstate is formed. Calcium tungstate precipitates from the aqueous solution and can be separated. The CaWO.sub.4 phosphors so produced, while brighter than the solid state reaction products, have had an objectionable persistence or lag.
It is believed, therefore, that a calcium tungstate X-ray phosphor composition which exhibits no detectable persistence and exhibits increased brightness is an advancement in the art. An additional advancement in the art is the capability of certain embodiments to achieve brightness considerably in excess of the prior art phosphors.